The underground benzene plume from the Sterling Zip Mart gasoline spill has appeared to stop growing, but the pollutants likely will be around for decades, Department of Environmental Conservation engineers told Sterling residents Monday night.
Meeting at the Sterling Baptist Church at the intersection of the Sterling Highway and Swanson River Road, about one-quarter mile from the site of the original spill, the engineers reported on cleanup progress and announced plans to install a vapor extraction system to capture benzene vapors.
To date, the DEC has recovered 14,500 gallons of gasoline through automated product recovery wells.
Due to some Zip Mart record keeping discrepancies, estimates of how much gasoline actually leaked into the ground vary from 50,000 to as high as 120,000 gallons.
In early 2002, an engineering firm hired by Zip Mart owner Whittier Properties Inc. found heavily contaminated soil and ground water. A foot of regular-grade gasoline was found floating on top of ground water in two monitoring wells at the site.
A DEC investigation revealed that a fill pipe was not properly connected to underground storage tanks at the gas station and much of the gasoline being delivered was not getting into the tanks, but was spilling into the surrounding ground.
Because the ground pollution threatened drinking water wells at the nearby Sterling Elementary School, Sterling Baptist Church, Sterling Lutheran Church and several private businesses, monitoring and recovery wells were drilled.
Free-standing liquid gasoline is being extracted from the ground on Sterling Community Club property next to the former gas station site. Monitoring wells are being used to track the down-gradient spread of the resultant benzene plume heading in a southeasterly direction beneath Sterling Baptist Church, Wedding Brokers and B&D Auto.
"We got the easy product out. Now we're pulling 30 to 50 gallons a month of liquid product," said Stafford Glashan, project manager for Shannon and Wilson Inc., an Anchorage engineering firm working with the DEC to clean up the spill.
One of about 80 people attending the community meeting Monday asked what residents in the affected area can or should do now.
"We need to look at each one individually," said Paul Horwath, environmental engineer for DEC.
"One opinion might be to drill a deeper (water) well; one might be (drinking) water treatment," he said.
Horwath also told the residents that some of the adjacent properties are actually up-gradient from the spill site and will not have their drinking water affected.
For those property owners, no action is needed right now, he said.
The next phase in the cleanup is installation of a vapor extraction system, according to Jim Frechione, environmental manager for the DEC contaminated sites program.
The system will remove subsurface gasoline vapors benzene using seven product recovery wells previously installed to recover the liquid gasoline.
The benzene vapors will be vacuumed from the ground and treated in a thermal oxidizer to be erected on Community Club grounds, if approved.
Vapors will be heated to 1,400 degrees Fahrenheit in the oxidizer, which initially will be able to treat up to 20 pounds of hydrocarbons per hour.
As the amounts of recovered benzene vapors diminish, the oxidizer will be converted to chemically alter the vapor molecules, turning them into harmless carbon dioxide and water, according to Glashan.
Residents will not see any flaming flare stacks like those typically seen at refineries and chemical plants, but may occasionally see plumes of steam, depending on weather conditions.
Equipment will be housed in small sheds erected on site, and low-horsepower blowers used to create the vacuum should not be noisy, according to Horwath.
"We need to look at getting money to put into a DEC account to help people with new (water) wells if needed, to ensure public health," said Sen. Tom Wagoner, who represents Sterling.
In addition to Wagoner, Rep. Mike Chenault and Kenai Peninsula Borough Assembly member Grace Merkes, who also represents Sterling, attended the meeting.
Horwath said the DEC would like the required two-week public comment period on the proposed vapor extraction system to begin now and said the department would work with the individual property owners as needed.
"We would like to start this fall," he said.
Bob Oakes, representing the Sterling Community Club, said he would discuss the plan with other club members.
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